The present invention relates to differential pair attenuation and control circuits and techniques, and more particularly to an attenuator control circuit and a temperature compensation circuit for controlling and stabilizing operation of a differential pair attenuator.
The bipolar junction transistor (BJT) differential pair is often used as the key element in an attenuator or automatic gain control (AGC) amplifier. The input signal, in the form of a current, is injected at the common emitters (emitters connected together) of the BJT differential pair. The output signal, also in the form of a current, is derived from the collector of one of the transistors. The difference in base voltages between the differential pair determines the ratio of output signal current to input signal current.
The attenuation function often needs to be linear in decibels (dB) and invariant with temperature and process variations. If the attenuator is to be xe2x80x9clinear in dBxe2x80x9d, then the collector current, referred to as IC, of one of the differential pair must vary (increase/decrease) exponentially with a linear change in control voltage. The collector current is constrained, however, to a maximum bias current, referred to as IBIAS, provided by a constant current sink coupled between the emitter terminals of the differential pair and ground. For some large positive value for the voltage differential between the base voltages of the differential pair, referred to as VD, the ratio of the output current to the input current is one-to-one (1:1). For a range of large negative VD, the current IC typically does vary exponentially with linear changes of VD. For small values of VD, however, the current does not respond exponentially. It is also noted that a thermal coefficient voltage, referred to as VT, gives the transfer function a temperature dependence. The thermal coefficient voltage VT is the voltage equivalent of temperature, where VT=kT/q, where xe2x80x9ckxe2x80x9d is the Boltzmann constant in joules per degree Kelvin, T is the temperature in degrees Kelvin (absolute scale), and xe2x80x9cqxe2x80x9d is the magnitude of the charge of an electron. Simply applying the input gain control voltage between the bases of the differential pair, therefore, does not result in a temperature independent, xe2x80x9clinear in dBxe2x80x9d response as desired.
An attenuator control circuit according to an embodiment of the present invention controls operation of a differential pair attenuator to provide linear in decibels (dB) operation. The differential pair attenuator includes first and second control input terminals that collectively receive a control voltage which is intended to control the attenuation of output current of the differential pair attenuator. In one configuration, an input current signal is injected at the common emitters of the differential pair and an output current signal is developed at the collector of one of the transistors. The output collector is coupled to a supply voltage through a resistor. The difference in base voltages between the differential pair determines the ratio of output signal current to input signal current. It is desired that the attenuation function be linear in dB and invariant with temperature and process variations. The differential pair attenuator alone, however, does not meet the desired attenuation function in certain circumstances, such as when the control voltage is small or large and positive. Also, the differential pair attenuator is dependent upon temperature and process variations.
The attenuator control circuit corrects for these deficiencies of the differential pair attenuator. The attenuator control circuit includes first and second transistors forming a control differential pair that is biased by a bias current. The control differential pair has first and second current paths and first and second current control terminals, where the first and second current control terminals are coupled to the respective first and second control input terminals of the attenuator differential pair. The attenuator control circuit also includes a current control circuit that sources a supply current to the first and second current paths of the control differential pair, where the supply current is approximately equal to the bias current. The attenuator control circuit also includes an amplifier that has an input coupled to the current control circuit and an output coupled to the second control terminal of the control differential pair. The amplifier controls current through the second current path of the control differential pair and attempts to maintain constant total current through the first and second current paths. In one embodiment, the constant total current through the first and second current paths is approximately the same as the bias current. Since the total current through the control differential pair is kept constant by controlling one current path of the control differential pair, the other current path of the control differential pair exhibits the desired exponential attenuation function. Since the control differential pair is coupled in parallel with the differential pair attenuator, the output current of the differential pair attenuator also operates according to the desired exponential attenuation function.
The current control circuit may include a bias current circuit and a current mirror. The current mirror has an input coupled to the bias current circuit and an output coupled to the first and second current paths of the control differential pair. In this manner, the current mirror applies the same current developed by the bias control circuit to the current paths of the control differential pair. In one embodiment, the amplifier is a non-inverting amplifier that has its input coupled to the output of the current mirror. In an alternative embodiment, the amplifier is an inverting amplifier that has its input coupled to the input of the current mirror. Operation is similar in either case. The attenuator bias current circuit may include a bias current sink and a third transistor, where the third transistor has a control terminal and first and second current terminals. The control terminal of the third transistor is coupled to the first current control terminal of the control differential pair. Also, the first current terminal is coupled to the input of the current mirror and the second current terminal is coupled to the bias current sink.
In a more specific embodiment, the first, second and third transistors are matched bipolar junction transistors having common emitters coupled to a bias current circuit that sinks approximately twice the bias current. Furthermore, the attenuator control circuit may include a temperature compensation circuit coupled between the control terminal of the third transistor and the first current control terminal of the control differential pair. The temperature compensation circuit is a suitable fixed bias voltage circuit that applies a temperature proportional voltage between the control terminal of the third transistor relative and the first current control terminal of the control differential pair. The temperature compensation circuit operates to counteract the temperature dependence of the attenuator control circuit and the differential pair attenuator.
In one embodiment, the temperature compensation circuit includes first and second differential-to-single-ended stages, each having a differential input and an output. A first polarity of the differential input of the first stage is coupled to a first polarity of the differential input of the second stage forming a feedback node. A reference signal is applied to a second polarity of the differential input of each of the first and second differential-to-single-ended stages. A temperature independent current sink is coupled to bias the first differential-to-single-ended stage and a temperature proportional current sink is coupled to bias the second differential-to-single-ended stage. Further, a current circuit is coupled to the output of the first differential-to-single-ended stage, where the current circuit draws a temperature independent current. An amplifier is provided with an input coupled to the output of the first differential-to-single-ended stage and an output coupled to the feedback node. Finally, the temperature compensation circuit includes an output circuit that applies the temperature proportional voltage.
In a more specific embodiment, an attenuator control circuit according to another embodiment includes first, second and third matched transistors coupled in a common emitter configuration. A current sink is coupled to the common emitters of the three transistors. The second and third transistors form a differential pair having first and second base terminals coupled to first and second base terminals, respectively, of the differential pair attenuator. The second transistor has a collector terminal that is coupled to a collector terminal of the third transistor. A current mirror is provided having a first current terminal coupled to a collector terminal of the first transistor and a second terminal coupled to the collector terminals of the second and third transistors. An amplifier has an input coupled to the current mirror and an output coupled to the base terminal of the third transistor. Finally, a voltage source is provided that applies a temperature proportional bias voltage between the base terminals of the first and second transistors.
The current mirror may include a first diode-coupled transistor coupled at its first current terminal to the collector terminal of the first transistor and a second transistor coupled at its second current terminal to the collector terminals of the second and third transistors of the common emitter configuration. In one embodiment, the amplifier is a non-inverting amplifier having its input coupled to the second current terminal of the current mirror. In an alternative embodiment, the amplifier is an inverting amplifier having its input coupled to the first current terminal of the current mirror.
The fixed bias voltage source may include first and second differential-to-single-ended stages, each having a differential input and an output. A first polarity of the differential input of the first stage is coupled to a first polarity of the differential input of the second stage at a feedback node. The fixed bias voltage source further includes a reference signal that is applied to a second polarity of the differential input of each of the first and second differential-to-single-ended stages. The fixed bias voltage source further includes a temperature independent current sink coupled to bias the first differential-to-single-ended stage, a temperature proportional current sink coupled to bias the second differential-to-single-ended stage, an attenuator input gain control current source coupled to the output of the first differential-to-single-ended stage, an amplifier having an input coupled to the output of the first differential-to-single-ended stage and an output coupled to the feedback node, and an output circuit coupled to the output of the second differential-to-single-ended stage that develops the fixed bias voltage.